I bought a little seedling in the store- had a jack-o-lantern tag in it. I've never grown them before so don't know what they look like but this thing looks more like a watermelon. Is this what pumpkins look like when they start out?? Thanks!

Yes, they definely do. Before it gets too big make sure there's enough drainable stuff under it so it doesn't rot. It looks like you have some straw there, but I can't tell how thick. A couple inches should do it. It'll compress, but that's okay.